If it is a sine wave oscillator you can drive a coil with it and when a piece of metal (one that induced currents, due to changing magnetic field, can be created inside) comes close to the coil, the current through the coil will decrease.

As the metal object gets closer to the coil its inductance increases and thus the current decreases (see equation above). Regarding the inductance, you have to study magnetics to see how it is affected by the material parameters, number of turns of the coil, etc.

In short: a metal detector consists of two oscillators,one the search coil[part of an oscillator] and a referance oscillator ,the two frequences are non linearly mixed.Oscillators are adjusted for O beat . When a metalic object comes near the search coil ,its inductance is changed causing a change in frequency,the beat note is herd as a tone.

In short: a metal detector consists of two oscillators,one the search coil[part of an oscillator] and a referance oscillator ,the two frequences are non linearly mixed.Oscillators are adjusted for O beat . When a metalic object comes near the search coil ,its inductance is changed causing a change in frequency,the beat note is herd as a tone.

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Commercially available BFO metal detectors are not common. More common are VLF (very low frequency) and PI (pulse induction) types.Here is a discussion of how they work. I haven't read it carefully, so I can't vouch for its accuracy.

lol... I actually in the middle - well, the end- of a project where me and my group have to make a metal detector , using one coil.
I can see it is an old thread, but hey..would love to share what I have ( have to hand in on may 26th09).

This is WEERD! I've noticed that questions about a certain subject come up shortly after an article on said subject has shown up in Nuts&Volts! And from the questions, it does not appear that the questioner has read the article.

Anyway, yep, there was a two-issue article on metal detecting in the March & April 2009 issues.

lol... I actually in the middle - well, the end- of a project where me and my group have to make a metal detector , using one coil.
I can see it is an old thread, but hey..would love to share what I have ( have to hand in on may 26th09).

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We'll look forward to seeing your design and a report on its performance.